Using the boron neutron capture therapy (BNCT) to treat liver cancer should be a valuable method of treatment, but it also needs to have at least the following advantages: non-invasive and without the side effects from the chemical treatments, has almost no adverse effects on the normal liver cells so as to allow for quick recovery in patients, but the aforesaid requirements could not be satisfied due to the lack of adequate boron drugs in the previous methods.
Before the present invention, the use of BNCT to treat cancer was mostly based on boron drugs that result in the highest ratio of boron concentrations between a tumor and the normal tissues (T/N ratio, the higher the better), such that only the tumor cells are selectively killed during the BNCT treatment. Currently, the human clinical trials using BNCT for treating cancer include boron drugs such as BPA (Boronophenylalanine) and BSH (Borocaptate sodium).
BSH is a water-soluble boron drug, and had been approved by the FDA to be used as a boron drug in the BNCT clinical trials. BSH is commonly used in the BNCT clinical trials for treating glioma patients. When the blood-brain barrier (BBB) functions normally, BSH is unable to penetrate this barrier, but because BBB is damaged by tumors and thus became incomplete in the glioma patients, BSH is able to penetrate the barrier and into the tumor cells.
BPA is also a boron drug that had been approved by the FDA for use in the BNCT clinical trials; it is water-soluble and an analog of phenylalanine. Because the expression of amino acid transporters on the membrane of tumor cells is different from that in the normal cells, the uptake of BPA by tumor cells was usually higher than that by normal cells. However, according to the previous animal models in which BPA was used as a boron drug, the pancreas adjacent to the liver had a higher concentration of boron accumulated therein [F. I. Chou, H. P. Chung, H. M. Liu, C, W. Chi, and W. Y. Lui, “Suitability of boron carriers for BNCT: accumulation of boron in malignant and normal liver cells after treatment with BPA, BSH and BA,” Applied Radiation and Isotopes vol. 67, pp, 105-108, 2009.]. Therefore, when neutron irradiation was carried out and the liver tumor had acquired sufficient treatment dose, the pancreas was severely damaged because of the neutron capture reaction. Therefore, when BPA is used as a boron drug in a BNCT clinical trial for treating a liver tumor, a patient's liver has to be removed by surgery after the injection of BPA, so as to carry out ex vivo neutron irradiation of the liver tumor. That is, the tumor-bearing liver cannot undergo neutron irradiation in vivo. But the ex vivo neutron irradiation has certain limitations and risks; for example, its success is dependent on a surgeon's organ removal technique, a patient's physical conditions, and sterilization throughout the surgery. In 2007, Suzuki and his colleagues had attempted the first case of using BNCT to treat primary liver cancer in a patient, in which BPA and BSH was injected into the liver by the hepatic artery, then BSH was mixed with lipiodol in order to carry out embolization. This method had significantly reduced the risks of the BNCT treatment process, but it failed to successfully treat the liver cancer in the patient. The patient had suffered a relapse and died of liver failure at 10 months after the BNCT treatment.
Boric acid (BA) has a molecular formula of H3BO3 and a molecular weight of 61.83 g/mol. Under normal physiological pH, it appears to be a uncharged small molecule and has a molecular volume of 71.5 Å3, which is even smaller than that of uric acid (75.3 Å3). Once the naturally occurring boron atoms dissolves in water, they exist as the weakly acidic boric acid. The B—O bonding in the boric acid is very stable so boric acid is nearly never metabolized in the living creatures. In addition, boric acid is widely applied in our daily lives and commonly used in pesticides, weed killers, and preservatives, thus its biological toxicity in humans, rats, and rabbits via oral intake had been thoroughly studied. In a living creature, boric acid is quickly and passively transported and diffused throughout the body and into cells along with the body fluids. In the early studies of accumulation of boron drugs in tumors and normal tissues, boric acid is frequently used as a reference compound, and previous studies had shown that boric acid has an accumulation efficiency of 1 in the cells. In the studies of cell culture, it was shown that boric acid was evenly distributed in the GS-9L, U-343MGa, HeLa, V79, and B16 cells [J. A. Coderrea and G. M. Morrisb, “The radiation biology of boron neutron capture therapy,” Radiation Research, vol. 151, pp. 1-18, 1999.], [J. Capala, M. S. Makar, and J. A. Coderre, “Accumulation of boron in malignant and normal cells incubated in vitro with boronophenylalanine, mercaptoborane or boric acid,” Radiation Research, vol. 146, pp. 554-60, 1996.].
After a tumor-bearing rat had been treated with the boron drug sulfhydryl borane dimer, the boron concentration in its blood was analyzed by using the Prompt-γ spectrometry, which utilizes boric acid as an internal control [D. D. Joel, R. G. Fairchild, J. A. Laissue, S. K. Saraf, J. A. Kalef-Ezra, and D. N. Slatkin, “Boron neutron capture therapy of intracerebral rat gliosarcomas,” Proceedings of the National Academy of Sciences of the United States of America, vol. 87, pp. 9808-9812, 1990.].
Studies based on using the electroporation method to increase the uptake of a boron drug (10B-BSH) in tumor cells revealed that when using boric acid as a reference for comparison, 10B-boric acid freely enters and leaves the cells, and the concentration of 10B in the cells and in the liquid culture was identical, and showed no accumulation in the cells.
According to the principle of BNCT treatment, the ratio of boron concentrations of a boron drug accumulated in the tumor and the normal tissues need to be higher than 2.5 to 3 [R. F. Barth, J. A. Coderre, M. G. H. Vicente, and T. E. Blue, “Boron neutron capture therapy of cancer: current status and future prospects,” Clinical cancer research, vol. 11, pp. 3987-4002, 2005.], such that the BNCT treatment can effectively kill the tumor cells, while damage to the normal tissues can be reduced to acceptable levels. As boric acid enters and leaves the cells by simple diffusion or passive diffusion, and is not selectively accumulated, it results in a ratio of boron concentrations of 1 intracellularly and extracellularly. Because boric acid does not result in any specific accumulation effects in the soft tissues of a living creature, it does not fulfill the requirement of specific accumulation of a boron drug in a tumor that the BNCT treatment is based on, thus boric acid is mostly used as an internal reference in the BNCT studies to test the distribution of a boron drug in a living creature, or as an internal standard for analyzing the boron concentration. There have not been any BNCT treatments that use boric acid as a boron drug until now.